The present invention relates to a process for recovering a nitrogen gas under a desired higher pressure than the pressure of a nitrogen gas withdrawn from an upper column of a duplex type rectification tower in an air separation apparatus.
A flow diagram of an air separation apparatus according to the conventional cryogenic separation process is shown. Raw material air is supplied to a heat exchanger 5 from a pipe 101, cooled by a low temperature return gas therein, and injected into a lower column 1 of duplex type rectification tower through a pipe 102. The raw material air is distilled therein and separated into pure nitrogen, impure nitrogen and liquid air rich in oxygen. The pure nitrogen is withdrawn from the lower column 1 and led to an upper column 3 through a pipe 109, an expansion valve 12 and a pipe 110 for use in the upper column 3 of duplex-type rectification tower as a reflux. The impure nitrogen is also withdrawn from the lower column 1 and led to the upper column 3 through a pipe 107, an expansion valve 11 and a pipe 108 for use in the upper column 3 as a reflux. The liquid air is supplied to the upper column 3 from the lower column 1 through a pipe 105, an expansion valve 10 and a pipe 106. On the other hand, a portion of the raw material air is withdrawn from the heat exchanger 5 at an intermediate position to compensate a coldness of the entire air separation apparatus, led to an expansion turbine 4 through a pipe 103, adiabatically expanded therein to generate coldness, and injected into the upper column 3 through a pipe 104. From the upper column 3 are withdrawn pure oxygen through a pipe 111, impure nitrogen through a pipe 113 and pure nitrogen through a pipe 115. They are individually heated in the heat exchanger 5 in heat exchange with the incoming raw material air, and pure oxygen is withdrawn through a pipe 112, impure nitrogen through a pipe 114 and pure nitrogen through a pipe 116.
The nitrogen gas withdrawn from the upper column 3 is under a pressure of about 200 mmAq at the outlet of the heat exchanger 5 owing to the pressure loss in the upper column 3. Thus, when the pressure of nitrogen is increased to about 1,000 mmAq in the apparatus of FIG. 1, the pressure of nitrogen in the lower column 1 must be increased to maintain the necessary temperatures of oxygen in the upper column 3 and nitrogen in the lower column 1 through a main condenser 2, and consequently the pressure of raw material must be increased. That is, a power for compressing the raw material air must be increased, and this is not economical.